Fluorine-containing elastomer and composition thereof for crosslinking

ABSTRACT

To provide a novel crosslinking system of fluorine-containing elastomer giving a crosslinked product particularly having improved mechanical strength and compression set at high temperature. A fluorine-containing rubber composition comprising a fluorine-containing elastomer having carboxyl group and/or alkoxycarbonyl group at an end of a trunk chain and/or branched chain as a crosslinkable group.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Divisional of application Ser. No. 10/726,604 filed Dec. 4,2003, now U.S. Pat. No. 7,309,743, which is a Divisional of applicationSer. No. 09/831,509 filed Sep. 28, 2001, now abandoned which is a 371 ofPCT Application No. PCT/JP99/06243 filed Nov. 10, 1999, the above-notedapplications incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a fluorine-containing elastomer givinga novel crosslinking system, a preparation process thereof and afluorine-containing rubber composition for crosslinking. According tothe present invention, a crosslinked fluorine-containing rubber moldedarticle being excellent in mechanical properties, particularlycompression set and heat resistance can be provided.

BACKGROUND ART

Fluorine-containing elastomers, particularly perfluoro elastomers mainlycomprising a tetrafluoroethylene (TFE) unit are widely used as a sealingmaterial to be used under hard environment because of excellent chemicalresistance, solvent resistance and heat resistance thereof.

However requirements for characteristics thereof required with advanceof technology are becoming strict. In the fields of space aeronautics,semi-conductor production apparatuses and chemical plants, a sealingproperty under high temperature environment of 300° C. or more isdemanded.

To cope with such requirements, proposals have been made to enhance heatresistance by improving a crosslinking system. As such a crosslinkingsystem, there are known crosslinking methods, for example, a method oftriazine crosslinking by forming a triazine ring with an organotincompound by using a fluorine-containing elastomer having a nitrile groupintroduced as a crosslinking point (for example, JP-A-58-152041), amethod of oxazol crosslinking by forming an oxazol ring withbisaminophenol similarly by using a fluorine-containing elastomer havinga nitrile group introduced as a crosslinking point (for example,JP-A-59-109546), a method of imidazole crosslinking by forming animidazole ring with a bisdiaminophenyl compound (for example,JP-A-59-109546) and a method of thiazole crosslinking by forming athiazole ring with bisaminothiophenol (for example, JP-A-8-104789).

In a series of PCT patent applications (WO97/19982, WO98/23653,WO98/23654 and WO98/23655) of Du Pont, U.S.A., it is proposed that anend of a fluorine-containing elastomer having nitrile group is convertedto a carbonyl-containing end group for the purpose to increase acrosslinking rate in peroxide crosslinking system in addition to theabove-mentioned triazine crosslinking system and oxazole crosslinkingsystem.

However a crosslinked rubber obtained by a crosslinking system describedin JP-A-58-152041, JP-A-59-109546 and JP-A-8-104789 is insufficient in amechanical strength and compression set at high temperature because acrosslinkable functional group is present only in a branched chainderived from a cure site monomer.

An essential object of the inventions disclosed in the above-mentionedapplications of Du Pont is to decrease the number of sulfonic acid endgroups which may cause a crosslinking failure. To achieve such anobject, the end groups are converted to carbonyl-containing groups.Namely, the crosslinking rate is enhanced by decreasing the number ofsulfonic acid groups but not by using carbonyl-containing groups ascrosslinking points. This can be seen from the description that it ispreferable to decarboxylate carbonyl-containing end groups by heating todecrease a viscosity of the elastomer because the carbonyl-containingend groups ionized or to be ionized increase the viscosity of elastomer.In those publications, carboxyl group, carboxylic acid salt andcarboxyamide group are raised as a carbonyl-containing end group.However since a metal salt is used for coagulation of the obtainedemulsified polymer, carbonyl-containing end groups of the coagulated andisolated elastomer to be subjected to crosslinking have been convertedto metal salts of carboxylic acid or carboxyamide group. It is assumedthat those salts cause an increase in a viscosity of the elastomer. Thedecarboxylation treatment of the carbonyl-containing end groupsindicates that the end groups are not used for crosslinking.

Further also in the crosslinking system of the invention of Du Pont, inwhich a fluorine- and nitrile-containing elastomer having acarbonyl-containing group as an end group is used, a mechanical strengthand compression set at high temperature of the obtained crosslinkedproduct are not improved.

An object of the present invention is to provide a novel crosslinkingsystem of a fluorine-containing elastomer giving a crosslinked producthaving improved mechanical strength and compression set at hightemperature.

DISCLOSURE OF INVENTION

Namely, the present invention relates to a fluorine-containing rubbercomposition for crosslinking comprising a fluorine-containing elastomerhaving carboxyl group and/or alkoxycarbonyl group at an end of a trunkchain and/or branched chain as a crosslinkable group and a crosslinkedfluorine-containing rubber molded article obtained by crosslinking thefluorine-containing rubber composition, particularly to thefluorine-containing rubber composition for crosslinking comprising afluorine-containing elastomer having carboxyl group as a crosslinkablegroup at an end of a trunk chain and/or branched chain, particularly atthe ends and the crosslinked fluorine-containing rubber molded articleobtained by crosslinking the fluorine-containing rubber composition

Further the present invention relates to a process for preparing thefluorine-containing elastomer which is characterized in that after afluorine-containing monomer is polymerized by using, as one of apolymerization initiator and/or monomer, a compound giving carboxylgroup and/or alkoxycarbonyl group to an end of a trunk chain and/orbranched chain, a polymerization product is treated with an acid.

Example of the preferred fluorine-containing elastomer is a novelcrosslinkable fluorine-containing elastomer which has carboxyl groupand/or alkoxycarbonyl group at an end of a trunk chain and/or branchedchain as a crosslinkable group and is represented by the formula (I):X¹-[A-(Y)_(p)]_(q)—X²  (I)or the formula (II):X¹-[A-(Y¹)_(p)]_(q)—[B—(Y²)_(r)]_(s)—X²  (II)wherein X¹ and X² are the same or different and each is carboxyl group,alkoxycarbonyl group, iodine atom, bromine atom or sulfonic acid group,Y, Y¹ and Y² are the same or different and each is a divalent organicgroup having carboxyl group, alkoxycarbonyl group, iodine atom, bromineatom or nitrile group at a side chain thereof, A is an elastomericfluorine-containing polymer chain segment (hereinafter referred to as“elastomeric segment A”), B is a non-elastomeric fluorine-containingpolymer chain segment (hereinafter referred to as “non-elastomericsegment B”), p is 0 or an integer of 1 to 10, q is an integer of 1 to 5,r is 0 or an integer of 1 to 10, s is an integer of 1 to 3, any one ofX¹, X², Y, Y¹ and Y² is carboxyl group or alkoxycarbonyl group, Y, Y¹and Y² may be contained at random in the segment A or B.

A preferred elastomeric fluorine-containing polymer chain segment is onecomprising not less than 90% by mole of a perhalo olefin unit.

The present invention also relates to the fluorine-containing elastomerhaving carboxyl group at an end of a trunk chain as a crosslinkablegroup and represented by the formula (Ia):X¹-[A-(Y)_(p)]_(q)—X²  (Ia)or the formula (IIa):X¹-[A-(Y¹)_(p)]_(q)—[B—(Y²)_(r)]_(s)—X²  (IIa)wherein X¹ and X² are the same or different and each is carboxyl group,alkoxycarbonyl group, iodine atom, bromine atom or sulfonic acid group,Y, Y¹ and Y² are the same or different and each is a divalent organicgroup having carboxyl group, alkoxycarbonyl group, iodine atom, bromineatom or nitrile group at a side chain thereof, A is an elastomericfluorine-containing polymer chain segment, B is a non-elastomericfluorine-containing polymer chain segment, p is 0 or an integer of 1 to10, q is an integer of 1 to 5, r is 0 or an integer of 1 to 10, s is aninteger of 1 to 3, any one of X¹ and X² is carboxyl group, Y, Y¹ and Y²may be contained at random in the segment A or B.

In the formulae (I), (II), (Ia) and (IIa), examples of an alkoxyl groupof the alkoxycarbonyl group are, for instance, linear or branchedalkoxyl groups having 1 to 10 carbon atoms. A part of hydrogen atoms maybe replaced by fluorine atoms.

Also it is preferable that the fluorine-containing elastomer of thepresent invention has a content of carboxyl group satisfying theequation (1):(Sco/Scf)×(D/Dp)×(F/Fp)≧0.01  (1)

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a chart of infrared spectroscopic analysis of thefluorine-containing elastomer of the present invention obtained inExample 1.

FIG. 2 is a chart of infrared spectroscopic analysis of thefluorine-containing elastomer of the present invention obtained inExample 2.

BEST MODE FOR CARRYING OUT THE INVENTION

First the crosslinkable fluorine-containing elastomer of the presentinvention is explained below.

The fluorine-containing elastomer to be used in the present invention isthe elastomer having the elastomeric segment A as a trunk chain andrepresented by the formula (I):X¹-[A-(Y)_(p)]_(q)—X²  (I)or the fluorine-containing multi-segment elastomer having a trunk chaincomprising the elastomeric segment A and the non-elastomeric segment Band represented by the formula (II):X¹-[A-(Y¹)_(p)]_(q)—[B—(Y²)_(r)]_(s)—X²  (II).In the formulae (I) and (II), X¹, X², Y, Y¹, Y², A, B, p, q, r and s areas defined above. Among them, the fluorine-containing elastomer havingcarboxyl group at an end of a trunk chain (namely, at least one of X¹and X² is carboxyl group) and represented by the above-mentioned formula(Ia) or (IIa) is a novel elastomer.

The elastomeric segment A may be, for example, perfluoro elastomersegments such as a copolymer rubber represented by the formula (1):

wherein m is 95 to 50, n is 5 to 50 in mol ratio, R_(f) is aperfluoroalkyl group having 1 to 8 carbon atoms and

-   a terpolymer rubber represented by the formula (2):

wherein 1 is 95 to 35, m is 0 to 30, n is 5 to 35 in mol ratio, R_(f) isa perfluoroalkyl group having 1 to 8 carbon atoms, or

-   non-perfluoro elastomer segments such as a copolymer rubber    represented by the formula (3):

wherein m is 85 to 60, n is 15 to 40 in mol ratio,

-   a terpolymer rubber represented by the formula (4):

wherein 1 is 85 to 20, m is 0 to 40, n is 15 to 40 in mol ratio,

-   a terpolymer rubber represented by the formula (5):

wherein 1 is 95 to 45, m is 0 to 10, n is 5 in mol ratio to 45, Z¹, Z²and Z³ are fluorine atom or hydrogen atom individually, R_(f) is aperfluoroalkyl group having 1 to 8 carbon atoms,

(l is 1 to 80, m is 0 to 80, n is 10 to 50 in mol ratio, R_(f) is asdefined above).

Preferable examples of Y, Y¹ and Y² for introducing a crosslinking pointin the branched chain are, for instance, iodine-containing monomersrepresented by CX₂═CX—R_(f) ¹CHRI, wherein X is H, F or CH₃, R_(f) ¹ isa linear or branched fluoro or perfluoro alkylene group, fluoro orperfluoro oxyalkylene group, fluoro polyoxyalkylene group or perfluoropolyoxyalkylene group which may have one or more ether type oxygenatoms, R is H or CH₃, bromine-containing monomers, nitrilegroup-containing monomers represented by:

wherein m is 0 to 5, n is 1 to 8,

wherein m is 1 to 5,

wherein n is 1 to 4,

wherein n is 2 to 5,

wherein n is 1 to 6,CF₂═CF[OCF₂CF(CF₃)]_(n)OCF₂CF(CF₃)X³wherein n is 1 or 2, or

wherein X³ is CN, COOH or COOR¹, in which R¹ is an alkyl group which mayhave fluorine atom of C1 to C10, carboxyl group-containing monomers,alkoxycarbonyl group-containing monomers, and the like. Usuallyiodine-containing monomers, nitrile group-containing monomers andcarboxyl group-containing monomers are suitable.

From the viewpoint of copolymerizability, examples of suitableiodine-containing monomer are perfluoro(vinyl ether) compounds, forinstance, perfluoro(6,6-dihydro-6-iodo-3-oxa-1-hexene),perfluoro(5-iodo-3-oxa-1-pentene), and the like.

In addition, there are fluoro vinyl ethers described in JP-B-5-63482 andrepresented by the formula:

wherein Y³ is trifluoromethyl group, n is 0 to 2.

Also CF₂═CHI, and the like can be used suitably.

When at least one of end groups X¹ and X² is carboxyl group oralkoxycarbonyl group, a unit having nitrile group or carboxyl group ispreferable from the viewpoint of crosslinkability.

The non-elastomeric segment B is not limited basically as far as thesegment contains fluorine atom and does not have the above-mentionedelastomeric property. The non-elastomeric segment B may be selecteddepending on characteristics and function intended to obtain bycopolymerization thereof. Particularly a crystalline polymer chainsegment having a crystalline melting point of not less than 150° C. ispreferable in order to impart mechanical properties.

Among the monomers being capable of constituting the non-elastomericsegment B, examples of the fluorine-containing monomer are, forinstance, one or two or more of perhalo olefins such as TFE,chlorotrifluoroethylene (CTFE), perfluoro(alkyl vinyl ether) (PAVE),hexafluoropropylene (HFP), CF₂═CF(CF₂)_(p)X, wherein p is an integer of1 to 10, X is F or Cl and perfluoro-2-butene; and partly fluorinatedolefins such as vinylidene fluoride, vinyl fluoride, trifluoroethylene,

wherein X⁴ and X⁵ are H or F, q is an integer of 1 to 10 andCH₂═C(CF₃)₂. Also one or two or more of monomers copolymerizabletherewith, for example, ethylene, propylene, vinyl chloride, vinylethers, carboxylic acid vinyl esters and acryls can be used ascopolymerizable components.

Among them, from the viewpoint of chemical resistance and heatresistance, preferable examples of the monomer to be used as a maincomponent are, for instance, a sole use of fluorine-containing olefin, acombination use of fluorine-containing olefins, a combination use ofethylene and TFE and a combination use of ethylene and CTFE.Particularly a sole use of perhalo olefin and a combination use ofperhalo olefins are preferable.

Examples thereof are

-   (1) VdF/TFE (0 to 100/100 to 0), particularly VdF/TFE (70 to 99/30    to 1), PTFE or PVdF;-   (2) ethylene/TFE/HFP (6 to 60/40 to 81/1 to 30),    3,3,3-trifluoropropylene-1,2-trifluoromethyl-3,3,3-trifluoropropylene-1/PAVE    (40 to 60/60 to 40);-   (3) TFE/CF₂═CF—R_(f) ³ (amount exhibiting non-elastomeric property,    namely, an amount of CF₂═CF—R_(f) ³ is not more than 15% by mole.    R_(f) ³ is a linear or branched fluoro or perfluoro alkyl group or    fluoro or perfluoro oxyalkyl group which may have at least one ether    type oxygen atom);-   (4) VdF/TFE/CTFE (50 to 99/30 to 0/20 to 1);-   (5) VdF/TFE/HFP (60 to 99/30 to 0/10 to 1);-   (6) ethylene/TFE (30 to 60/70 to 40);-   (7) polychlorotrifluoroethylene (PCTFE);-   (8) ethylene/CTFE (30 to 60/70 to 40);    and the like. Among them, from the viewpoint of chemical resistance    and heat resistance, non-elastomeric copolymers, particularly PITFE    and TFE/CF₂═CF—R_(f) ³ (R_(f) ³ is as defined above) are preferable.

Also the above-mentioned unit Y² giving curing sites may be introducedfor various crosslinking systems as a monomer being capable ofconstituting the non-elastomeric segment B in an amount of not more than5% by mole, preferably not more than 2% by mole.

A block copolymerization of the non-elastomeric segment B can be carriedout, for example, by changing to the monomer for the non-elastomericsegment B subsequently to emulsion polymerization of the elastomericsegment A.

A number average molecular weight of the non-elastomeric segment B canbe adjusted in a wide range of 1,000 to 1,200,000, preferably 3,000 to400,000.

The non-elastomeric segment B can be surely block-copolymerized with theelastomeric segment A when the elastomeric segment A comprises not lessthan 90% by mole, particularly not less than 95% by mole of perhaloolefin unit as a component unit thereof. And also a molecular weight(degree of polymerization) of the non-elastomeric segment B can beincreased.

As mentioned above, X¹ and X² which are end groups of the elastomer arecarboxyl group, alkoxycarbonyl group, iodine atom, bromine atom orsulfonic acid group. Example of the method for introducing such afunctional group to the ends of elastomer is a method of treating withan acid which is described hereinafter.

A feature of the present invention is to use fluorine-containingelastomers in which at least one of X¹, X², Y, Y¹ and Y² which canbecome a crosslinking point is carboxyl group or alkoxycarbonyl group.Among them, as mentioned above, the fluorine-containing elastomerrepresented by the formula (Ia) or (IIa) and having carboxyl group at anend of a trunk chain thereof (namely, at least one of X¹ and X² iscarboxyl group) is a novel elastomer.

The above-mentioned patent publications of Du Pont discloses afluorine-containing elastomer having a carbonyl-containing end group,and carboxyl group, carboxylic acid salt and carboxyamide group areraised therein as the carbonyl group-containing group. However in thepatent publications, those groups are treated as a carbonylgroup-containing group collectively without being discriminated fromeach other, and it is unknown whether or not carboxyl group is actuallypresent on the end group of a trunk chain. According to knowledge of theinventors of the present invention, under the polymerization conditionsand coagulation conditions, particularly under condition of a pH valueof 3.5 to 7.0 which are described in those International PatentPublications, the end group is substantially a metal salt or ammoniumsalt of carboxyl group, and it is assumed that not only alkoxycarbonylgroup but also free carboxyl group are scarcely present.

Another feature of the present invention is that the fluorine-containingelastomer of the present invention is not a polymerization productitself but a fluorine-containing elastomer isolated from a reactionproduct mixture of the polymerization. Therefore if a crosslinking agentis added to the elastomer or high energy rays are irradiated thereto,the elastomer is in a state of so-called a mass being crosslinkable.

As mentioned above, a polymerization reaction product (elastomer)obtained by polymerization represented by emulsion polymerization isusually subjected to crosslinking after isolated from the reactionproduct mixture of the polymerization by coagulation with a metal salt.Therefore even if the polymerization product contains carboxyl group,the carboxyl group is converted to a carboxylic acid salt at the stageof salting out, and a fluorine-containing elastomer having free carboxylgroup has not yet been obtained. For that reason, enhancement ofmechanical strength and compression set of a crosslinked product hasbeen inhibited.

As mentioned above, there have been no fluorine-containing elastomersrepresented by not only the above-mentioned formulae (Ia) and (IIa) butalso the formulae (I) and (II) being in a crosslinkable state.

From the viewpoint of securing crosslinking points to enhance heatresistance and compression set, it is preferable that a content ofcarboxyl group in the novel fluorine-containing elastomers of thepresent invention represented by the formulae (Ia) and (Ia) satisfiesthe following equation (1):(Sco/Scf)×(D/Dp)×(F/Fp)≧0.01

Then the abbreviations in the equation (1) are explained below.

Sco, Scf, D, Dp, F and Fp are the following respective values of theaimed fluorine-containing elastomer of the present invention and astandard perfluoro elastomer mentioned below.

-   Sco: Total area of absorbances at the absorptions derived from    carbonyl group of associated and non-associated carboxyl groups    having the absorption peaks at from 1,680 to 1,830 cm⁻¹ when    measurement is made with FT-IR with respect to the elastomer to be    measured. For example, in case of TFE/perfluoro(methyl vinyl ether)    (PMVE)/CF₂═CFOCF₂CF(CF₃)OCF₂CF₂X, in which X is CN or COOH, an    absorption derived from the associated carbonyl group appears at    1,800 to 1,820 cm⁻¹ and an absorption derived from the    non-associated carbonyl group appears at 1,760 to 1,780 cm⁻¹.-   Scf: Area of absorbance at absorption derived from a harmonic sound    of C—F bond having an absorption peak at from 2,220 to 2,840 cm⁻¹    when measurement is made with FT-IR with respect to the elastomer to    be measured. In case where nitrile group is present, Scf is a value    obtained by subtracting an area of absorbance at absorption derived    from nitrile group having an absorption peak at from 2,220 to 2,300    cm⁻¹ from a total area of absorbance at whole absorption having a    peak at from 2,220 to 2,840 cm⁻¹. This correction is made to    eliminate an influence of absorption derived from nitrile group    since a peak of the absorption appears at from 2,220 to 2,300 cm⁻¹    when nitrile group is present.-   D: Specific gravity at 20° C. of the elastomer to be measured.-   Dp: Specific gravity (measured value: 2.03) at 20° C. of a standard    perfluoro elastomer (copolymer of TFE/PMVE in a mole ratio of 58/42,    measured with ¹⁹F-NMR). The reason why a copolymer comprising    TFE/PMVE in a mole ratio of 58/42 is used as a standard perfluoro    elastomer is that it is easily obtainable.-   F: Fluorine content (% by weight) of the elastomer to be measured    obtained by elemental analysis.-   Fp: Fluorine content (measured value: 71.6% by weight) of the    above-mentioned standard perfluoro elastomer obtained by elemental    analysis.

Then explained below is a meaning which the equation (1) has.

The term Sco/Scf is a proportion of carbonyl group (carbonyl group ofcarboxyl group, hereinafter the same) to C—F bond in thefluorine-containing elastomer. In case where the fluorine-containingelastomer of the present invention is a perfluoro elastomer, only thisterm may be used. Namely, Sco/Scfp≧0.01 (Scfp: Area absorbance of C—Fbond of perfluoro elastomer).

The terms D/Dp and F/Fp are those used for correction in case where thefluorine-containing elastomer of the present invention is obtained bycopolymerizing a non-perfluoro elastomer, for example, vinylidenefluoride. Namely, when a non-perfluoro elastomer such as vinylidenefluoride is copolymerized, an amount of C—F bonds in the elastomerdecreases relatively and an area of absorbance of C—F bonds measuredwith transmission IR analyzer decreases relatively.

In general in case where the measurement is made with a transmission IR,an area absorbance of C—F is proportional to the number of moles offluorine atoms per a unit area of the elastomer (a value obtained bydividing a weight of fluorine atom by an atomic weight 19 of fluorine).Thereby a weight of perfluoro elastomer having a volume V is V×Dp (Dp isa specific gravity of perfluoro elastomer), and thus a weight offluorine in the perfluoro elastomer is V×Dp×Fp/100 (Fp is a fluorinecontent of the perfluoro elastomer (Fp % by weight)) and the number ofmoles of fluorine is V×Dp×Fp/1900. Similarly the number of moles offluorine in a non-perfluoro elastomer having a volume V is V×D×F/1900 (Dand F are a specific gravity and fluorine content of non-perfluoroelastomer, respectively).

Provided that an area of absorbance of C—F bond of the perfluoroelastomer is Scfp and an area of absorbance of C—F bond of thenon-perfluoro elastomer is Scf, since the area of absorbance of C—F bondis proportional to the number of moles, an equation Scfp/Scf=VDpFp/VDFis obtained and thus an equation Scfp=(DpFp/DF)×Scf is obtained. Whenthis equation is substituted for the equation Sco/Scfp≧0.01 of theperfluoro elastomer, the above-mentioned equation (1) can be obtained.

In the equation (1), the following measuring methods and apparatuses areused.

(FT-IR Measurement)

-   -   Measuring apparatus: FT-IR Spectro Meter Model 1760X available        from Perkin Elmer Co., Ltd.    -   Sample: About 0.1 mm thick film    -   Measuring conditions: Resolution 2 cm⁻¹, Data interval 1 cm⁻¹,        measured by transmission method        (Elemental Analysis)    -   Measuring apparatus: Micro Processor Ionalyzer Model 901        available from Orion Research Co., Ltd.    -   Measuring method: A small amount of Na₂O₂ (combustion improver)        is added to 1.4 to 1.9 mg of the sample, followed by combustion        in a combustion flask containing 25 ml of pure water. After        allowing to stand for 30 minutes, 10 ml is sampled and thereto        is added 10 ml of a solution (10 liter of a solution comprising        500 ml of acetic acid, 500 g of sodium chloride, 5 g of        tri-sodium citrate dihydrate, 320 g of sodium hydroxide and pure        water). Then an amount of F ion is measured with F ion meter.        (Specific Gravity)    -   Measuring apparatus: Automatic densimeter Model D-1 available        from Kabushiki Kaisha Toyo Seiki Seisakusho    -   Measuring condition: 20° C.

The equation (1) means that the fluorine-containing elastomer of thepresent invention, irrespective of perfluoro elastomer or non-perfluoroelastomer, contains not less than 1 mmol of carboxyl group per 1 kg ofthe polymer. Particularly preferable is 10 to 250 mmol. When theelastomer is prepared by copolymerizing a carboxyl group-containingmonomer, it is preferable that a copolymerized proportion of thecarboxyl group-containing monomer is from 0.3 to 2% by mole.

From the viewpoint of enhancement of physical properties of the obtainedcrosslinked product, carboxyl groups are preferably bonded to end groupsX¹ and X² of a trunk chain.

The fluorine-containing elastomer of the present invention can beprepared by polymerization methods such as emulsion polymerization,suspension polymerization and solution polymerization.

It is preferable to use a polymerization initiator which makes itpossible that carboxyl groups or groups being capable of formingcarboxyl group (for example, acid fluoride, acid chloride, CF₂OH, any ofwhich form carboxyl group in the presence of water) are present at anend of the elastomer. Examples thereof are ammonium persulfate (APS) andpotassium persulfate (KPS).

Also a chain transfer agent which is usually used for adjusting amolecular weight may be used. However a use thereof may be avoided asfar as possible because a proportion of groups being capable of formingcarboxyl groups or alkoxycarbonyl groups to be introduced into the endsof the elastomer is reduced. This does not apply to the case where thepolymerization initiator can make it possible that the above-mentionedgroups are present at the ends of elastomer. When the chain transferagent is not used, a molecular weight of the elastomer may be adjustedby carrying out the polymerization at a low pressure, for example, at apressure less than 2 MPa·G, preferably a pressure of not more than 1MPa·G. Other polymerization conditions are not limited particularly.However when producing a polymerization product having carboxyl group atan end and/or branched chain thereof without acid treatment explainedbelow, it is preferable that a pH value of a polymerization system isadjusted to a strong acid having a pH value of not more than 3.

Among the so-obtained polymerization products, some of them do notcontain free carboxyl group depending on the polymerization conditions.However by the acid treatment mentioned below, conversion to freecarboxyl group can be carried out.

One of major features of the present invention is to carry out an acidtreatment of a polymerization product to convert groups such as a metalsalt and ammonium salt of carboxylic acid being present therein tocarboxyl group. Examples of the proper acid treatment method are amethod of washing the polymerization product, for example, withhydrochloric acid, sulfuric acid, nitric acid, or the like and a methodof adjusting a system of a mixture after the polymerization reaction toa pH value of not more than 3 with such an acid.

It is preferable from the viewpoint of reduction of steps that the acidtreatment is applied as means for coagulation when isolating thepolymerization product from the polymerization reaction mixture bycoagulation. The polymerization product may be isolated by means offreeze drying, or the like after the acid treatment of thepolymerization mixture. Further a coagulation method by ultrasonic waveor by mechanical force may be employed.

Also carboxyl group can be introduced by oxidizing a fluorine-containingelastomer containing iodine or bromine with a fuming nitric acid.

Further the present invention relates to the fluorine-containing rubbercomposition containing the fluorine-containing elastomer having carboxylgroup and/or alkoxycarbonyl group at an end of a trunk chain and/orbranched chain thereof as crosslinkable group, particularly thefluorine-containing elastomer represented by the formula (I) or (II).

The fluorine-containing rubber composition of the present invention canbe crosslinked by a method of crosslinking without using a crosslinkingagent, for example, high energy ray irradiation methods such as electronbeam irradiation method, radiation method and ultraviolet rayirradiation method. However preferably a crosslinking agent reactablewith carboxyl group or alkoxycarbonyl group, particularly a crosslinkingagent which is used in oxazole crosslinking system, imidazolecrosslinking system and thiazole crosslinking system is blended.

Examples of the crosslinking agent which is used in oxazole crosslinkingsystem, imidazole crosslinking system and thiazole crosslinking systemare, for instance, bisdiaminophenyl crosslinking agent represented bythe formula (III):

wherein R³ is —SO₂—, —O—, —CO—, an alkylene group having 1 to 6 carbonatoms, a perfluoroalkylene group having 1 to 10 carbon atoms or a singlebond, one of R¹ and R² is NH₂ and another one is —NH₂, —OH or —SH andpreferably each of R¹ and R² is —NH₂, bisaminophenol crosslinking agent,bisaminothiophenol crosslinking agent, bisamidrazone crosslinking agentrepresented by the formula (IV):

(wherein R³ is as defined above,

bisamidoxime crosslinking agent represented by the formula (V) or (VI):

wherein R_(f) ³ is a perfluoroalkylene group having 1 to 10 carbonatoms,

wherein n is an integer of 10 to 10, and the like. Those bisaminophenolcrosslinking agent, bisaminothiophenol crosslinking agent andbisdiaminophenyl crosslinking agent have been used for a crosslinkingsystem employing nitrile group as a crosslinking point, but are reactedwith carboxyl group or alkoxycarbonyl group which thefluorine-containing elastomer of the present invention has to form anoxazole ring, thiazole ring or imidazole ring and give a crosslinkedproduct.

Particularly preferable crosslinking agents are compounds having aplurality of 3-amino-4-hydroxyphenyl groups, 3-amino-4-mercaptophenylgroups or 3,4-diaminophenyl groups represented by the formula:

wherein R³ is as defined above. Examples thereof are, for instance,2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (general term”bis(aminophenol) AF),2,2-bis(3-amino-4-mercaptophenyl)hexafluoropropane, tetraamino benzene,bis-3,4-diaminophenylmethane, bis-3,4-diaminophenylether,2,2-bis(3,4-diaminophenyl)hexafluoropropane, and the like.

An amount of the crosslinking agent is preferably from 0.1 to 10 partsby weight based on 100 parts by weight of the elastomer.

To the composition of the present invention can be added additives whichare usually added to a fluorine-containing elastomer composition as casedemands, for example, a filler, processing aid, plasticizer, coloringagent, and the like. In addition to the above-mentioned additives, oneor more of usual crosslinking agent and crosslinking accelerator otherthan the above-mentioned ones may be blended. Also a knownfluorine-containing rubber may be mixed in a range not lowering aneffect of the present invention.

The composition of the present invention can be prepared by mixing eachof the above-mentioned components by using usual processing equipmentfor rubber, for example, an open roll, Banbury mixer, kneader, or thelike. In addition, the composition can be prepared also by a method ofusing a closed mixer and a method of co-coagulation through emulsionmixing.

For producing a pre-molded article from the abovementioned composition,usual known methods may be employed, such as a method of heating andcompressing in a metal mold, a method of putting in a heated metal moldunder pressure or a method of extruding with an extruder. In cases ofextruded products such as a hose and cable, since shapes thereof can bemaintained even after the extruding, a pre-molded article extrudedwithout using a crosslinking agent can be used as it is. Of course, itis possible to use a pre-molded article subjected to crosslinking byheating with steam by using a crosslinking agent. Also in case of ashaped product such as O-ring, when it is difficult to hold a shape ofthe product in an un-crosslinked state after mold-releasing, it ispossible to maintain the shape by using a pre-molded article crosslinkedpreviously by using a crosslinking agent.

In the present invention when carrying out oxazole crosslinking by usinga crosslinking agent such as bisaminophenol, the crosslinking can becarried out under usual crosslinking conditions for fluorine-containingrubbers. For example, a crosslinked rubber can be obtained by putting afluorine-containing elastomer in a metal mold, carrying outpress-crosslinking by holding under pressure at 120° to 250° C. for 1 to60 minutes and then crosslinking in an oven by holding at 120° to 320°C. for 0 to 48 hours. Also to formulations of known crosslinking methodsfor fluorine-containing rubber such as polyamine crosslinking, polyolcrosslinking and peroxide crosslinking, a crosslinking agent such asbis(aminophenol) AF is co-used to crosslink the rubber.

Also imidazole crosslinking in which carboxyl group is crosslinked withbisdiaminophenyl crosslinking agent is optimum for a polymer containingcarboxyl group at other point than end thereof, and gives a crosslinkedproduct having good physical properties at a relatively low crosslinkingtemperature (for example, 150° to 230° C., preferably 170° to 200° C.).

The present invention also relates to the so-obtained crosslinkedproduct. In the crosslinked product of the present invention, since eventhe end group of the fluorine-containing elastomer can be crosslinked ascrosslinking point, a high mechanical strength which has not beenobtained can be provided. Furthermore it is a surprising fact that acompression set which is an index for evaluating a sealing propertyessential particularly for a sealing material is greatly reduced andalso a compression set at high temperature is reduced.

The crosslinked product of the present invention is useful as variousmolded articles in the fields shown in Tables 1, 2 and 3 on thefollowing pages.

TABLE 1 Field of industry Sub-field of industry Final product EquipmentParts Electrical Semi-conductor Semi-conductor CVD equipment O (square)ring, packing sealing material, tube, roll production apparatuses Liquidcrystal panel Dry etching equipment Coating, lining, gasket, diaphragm,hose production apparatus Plasma panel production Wet etching equipmentapparatus Oxidation and diffusion equipment Sputtering equipment Ashingequipment Cleaning equipment Ion implantation equipment TransportationVehicle Car Engine and auxiliary Gasket, shaft seal, valve stem seal,sealing material, equipment hose AT Hose, sealing material Fuel line andauxiliary O (square) ring, tube, packing, core material of valve,equipment hose, sealing material, diaphragm Aircraft Aircraft Fuel lineDiaphragm, O (square) ring, valve, tube, packing, hose, sealing materialRocket Rocket Fuel line Diaphragm, O (square) ring, valve, tube,packing, hose, sealing material Ship Ship Fuel line Diaphragm, O(square) ring, valve, tube, packing, hose, sealing material ChemicalChemical products Plant Production line of Lining, valve, packing, roll,hose, diaphragm, chemicals such as O (square) ring, tube, sealingmaterial pharmaceutical, agricultural chemical, coating and resin(Petroleum) Chemicals Pharmaceuticals Plug for chemicals Plug forchemicals Mechanical Photograph Developing machine Film developingmachine Roll X-ray film developing Roll machine Printing Printingmachine Printing roll Roll Coating Coating facilities Coating roll RollAnalyzer and Tube physical and chemical appliances Food Plant Foodprocessing line Lining, valve, packing, roll, hose, diaphragm, O(square) ring, tube, sealing material Metal Steel making Steel sheetprocessing Steel sheet processing roll Roll facilities

TABLE 2 Field of Industry Characteristics Required Electrical Plasmaresistance, acid resistance, alkali resistance, amine resistance, ozoneresistance, gas resistance, chemical resistance, cleanliness, heatresistance Transportation Heat resistance, amine resistance Heatresistance, amine resistance Fuel resistance, fuel permeability, heatresistance Fuel resistance, fuel permeability, heat resistance Fuelresistance, fuel permeability, heat resistance Fuel resistance, fuelpermeability, heat resistance Chemical Chemical resistance, solventresistance, heat resistance Chemical resistance, solvent resistance,heat resistance Cleanliness Mechanical Chemical resistance Chemicalresistance Solvent resistance Solvent resistance Food Chemicalresistance, solvent resistance, heat resistance Metal Heat resistance,acid resistance

TABLE 3 Field of industry Parts Electrical O ring and sealing materialfor gate valve of corresponding product or equipment O ring and sealingmaterial for quartz window of corresponding product or equipment O ringand sealing material for chamber of corresponding product or equipment Oring and sealing material for gate of corresponding product or equipmentO ring and sealing material for bell jar of corresponding product orequipment O ring and sealing material for coupling of correspondingproduct or equipment O ring and sealing material for pump ofcorresponding product or equipment O ring and sealing material for gascontroller for semi-conductor of corresponding product or equipment Oring and sealing material for resist developing and releasing solutionsO ring and sealing material for wafer cleaning solution Diaphragm ofpump for corresponding production equipment Hose for resist developingand releasing solutions Hose and tube for wafer cleaning solution Rollfor transferring wafer Lining and coating of tanks for resist developingand releasing solutions Lining and coating of tanks for wafer cleaningsolution Lining and coating of tanks for wt etching TransportationEngine head gasket Metal gasket Crank shaft seal Cam shaft seal Valvestem seal Manifold packing Oil hose Seal for oxygen sensor ATF hoseInjector O ring Injector packing O ring and diaphragm for fuel pump Fuelhose Chemical Mechanical Developing roll Developing roll Gravure rollGuide roll Gravure roll for magnetic tape production and coating lineGuide roll for magnetic tape production and coating line Various coatingrolls Food Metal

Particularly the crosslinked product of the present invention can beused built-in the following semiconductor manufacturing equipment.

(1) Etching System

-   -   Dry etching equipment        -   Plasma etching machine        -   Reactive ion etching machine        -   Reactive ion beam etching machine        -   Sputter etching machine        -   Ion beam etching machine    -   Wet etching equipment    -   Ashing equipment        (2) Cleaning System    -   Dry etching cleaning equipment        -   UV/O₃ cleaning machine        -   Ion beam cleaning machine        -   Laser beam cleaning machine        -   Plasma cleaning machine        -   Gas etching cleaning machine    -   Extractive cleaning equipment        -   Soxhlet extractive cleaning machine        -   High temperature high pressure extractive cleaning machine        -   Microwave extractive cleaning machine        -   Supercritical extractive cleaning machine            (3) Exposing System    -   Stepper    -   Coater and developer        (4) Polishing System    -   CMP equipment        (5) Film Forming System    -   CVD equipment    -   Sputtering equipment        (6) Diffusion and Ion Implantation System    -   Oxidation and diffusion equipment    -   Ion implantation equipment

The present invention is then explained by means of examples, but is notlimited to them.

EXAMPLE 1

A 3-liter stainless steel autoclave having no ignition source wascharged with 1 liter of pure water, 10 g of an emulsifying agent;

and 0.09 g of disodium phosphate.12H₂O as a pH control agent. After theinside of a system was replaced sufficiently with nitrogen gas anddeairing was carried out, the autoclave was heated to 50° C. withstirring at 600 rpm, and a gas mixture of tetrafluoroethylene(TFE)/perfluoro(methyl vinyl ether) (PMVE) (TFE/PMVE=25/75 in moleratio) was introduced so that the inside pressure would become 0.78MPa·G. Then 10 ml of an aqueous solution of ammonium persulfate (APS)having a concentration of 527 mg/ml was introduced with pressurizednitrogen gas to initiate a reaction.

With the advance of the polymerization, at the time when the insidepressure was lowered to 0.69 MPa·G, 3 g of CF₂═CFOCF₂CF(CF₃)OCF₂CF₂CN(CNVE) was introduced with pressurized nitrogen. Then 4.7 g of TFE and5.3 g of PMVE were introduced at the respective self-pressures so thatthe inside pressure would become 0.78 MPa·G. Thereafter with the advanceof the reaction, pressurized TFE and PMVE were introduced similarly.Thus increasing and decreasing of the inside pressure were repeatedbetween 0.69 MPa·G and 0.78 MPa·G, and in addition, at the time when atotal amount of TFE and PMVE reached 70 g, 130 g, 190 g and 250 g,respectively, 3 g of CNVE was introduced with pressurized nitrogen.

Nineteen hours after starting of the polymerization, when a total amountof TFE and PMVE reached 300 g, the autoclave was cooled and un-reactedmonomer was released to give 1,330 g of aqueous dispersion having asolid concentration of 21.2% by weight.

Then 1,196 g of the obtained aqueous dispersion was diluted with 3,588 gof water and added slowly to 2,800 g of an aqueous solution of 3.5% byweight of hydrochloric acid with stirring. After the addition, stirringwas carried out for five minutes and a coagulated product was filtrated.The obtained polymer was put in 2 kg of HCFC-141b, followed by stirringfor five minutes and filtrating again. After that, washing withHCFC-141b and filtration were repeated four times and vacuum drying wascarried out at 60° C. for 72 hours to give 240 g of polymer (nitrilegroup-containing elastomer).

As a result of ¹⁹F-NMR analysis, the obtained polymer was a polymercomprising TFE/PMVE/CNVE=56.6/42.3/1.1% by mole. Also as a result ofmeasurement by infrared spectroscopic analysis, a chart shown in FIG. 1was obtained.

In the chart of FIG. 1, a characteristic absorption of carboxyl groupwas recognized around 1,774.9 cm⁻¹ and 1,808.6 cm⁻¹ and a characteristicabsorption of OH group was recognized around 3,557.5 cm⁻¹ and 3,095.2cm⁻¹. A value obtained from the equation (Sco/Scf)×(D/Dp)×(F/Fp) was0.040.

When an elastomer obtained by coagulation of the obtained product withmagnesium chloride and ethanol was subjected to IR analysis forreference purpose, an absorption derived from carboxyl group was notpresent and an absorption of magnesium salt of carboxylic acid wasrecognized at 1,729 cm⁻¹.

Further when the coagulation was carried out by freeze coagulation (pH:3.5 to 7.0) and the obtained elastomer was subjected to IR analysissimilarly, an absorption of carboxyl group was not present and anabsorption of ammonium salt (—COONH₄) of carboxylic acid was recognizedat 1,651 cm⁻¹.

The obtained fluorine-containing elastomer (nitrile-containing elastomerhaving carboxyl groups at ends thereof) of the present invention,2,2-bis(3-amino-4-hydroxyphenyl) hexafluoropropane (bis(aminophenol) AF)as a crosslinking agent and carbon black (Thermax N-990 available fromCancarb Co., Ltd.) as a filler were mixed in a weight ratio of 100/2/20,and the mixture was kneaded with an open roll to give a crosslinkablefluorine-containing rubber composition.

After the fluorine-containing rubber composition was subjected topressing at 180° C. for 10 minutes for crosslinking, further atwo-staged crosslinking in an oven was carried out under the conditionsmentioned in Table 4. Thus a 2 mm thick crosslinked product and a sampleO-ring (AS-568A-214) were produced. The results of measurements of thecrosslinked product with respect to crosslinkability, physicalproperties in ordinary state and compression set are shown in Table 4.

(Crosslinkability)

Vulcanization curves of each composition for crosslinking were obtainedat a temperature shown in Table 4 with JSR Curastometer Model II andthen a minimum viscosity (ν min), maximum viscosity (ν max), inductiontime (T₁₀) and optimum vulcanization time (T₉₀) were obtained.

(Physical Properties in Ordinary State)

A 100% modulus, tensile strength, tensile elongation and hardness (JISHardness A) of a 2 mm thick crosslinked product in ordinary state (25°C.) were measured according to JIS K6301.

(Compression Set)

A compression set of the O-ring (AS-568A-214) was measured according toJIS K6301 after allowing to stand at 200° C. for 70 hours, 200° C. for168 hours, 230° C. for 70 hours and 230° C. for 168 hours, respectively.

EXAMPLE 2

A 3-liter stainless steel autoclave having no ignition source wascharged with 1 liter of pure water, 10 g of an emulsifying agent;

and 0.09 g of disodium phosphate.12H₂O as a pH control agent. After theinside of a system was replaced sufficiently with nitrogen gas anddeairing was carried out, the autoclave was heated to 50° C. withstirring at 600 rpm, and a gas mixture of tetrafluoroethylene(TFE)/perfluoro(methyl vinyl ether) (PMVE) (TFE/PMVE=25/75 in moleratio) was introduced so that the inside pressure would become 0.78MPa·G. Then 10 ml of an aqueous solution of ammonium persulfate (APS)having a concentration of 527 mg/ml was introduced with pressurizednitrogen gas to initiate a reaction.

With the advance of the polymerization, at the time when the insidepressure was lowered to 0.69 MPa·G, 3.78 g ofCF₂═CFOCF₂CF(CF₃)OCF₂CF₂COOH (CBVE) was introduced with pressurizednitrogen. Then 4.7 g of TFE and 5.3 g of PMVE were introduced at therespective self-pressures so that the inside pressure would become 0.78MPa·G. Thereafter with the advance of the reaction, pressurized TFE andPMVE were introduced similarly. Thus increasing and decreasing of theinside pressure were repeated between 0.69 MPa·G and 0.78 MPa·G. 4.2Hours after starting of the polymerization reaction, when a total amountof TFE and PMVE reached 80 g, the autoclave was cooled and un-reactedmonomer was released to give 1,091 g of an aqueous dispersion having asolid concentration of 7.5% by weight.

Then 1,000 g of the obtained aqueous dispersion was diluted with 3,000 gof water and added slowly to 2,800 g of an aqueous solution of 3.5% byweight of hydrochloric acid with stirring. After the addition, stirringwas carried out for five minutes and a coagulated product was filtrated.The obtained polymer was put in 800 g of HCFC-141b, followed by stirringfor five minutes and filtrating again. After that, washing withHCFC-141b and filtration were repeated four times and vacuum drying wascarried out at 120° C. for 72 hours to give 72 g of polymer.

As a result of ¹⁹F-NMR analysis, the obtained polymer was a polymercomprising TFE/PMVE/CBVE=57.3/41.8/0.9% by mole. Also as a result ofmeasurement by infrared spectroscopic analysis, a chart shown in FIG. 2was obtained.

In the chart of FIG. 2, a characteristic absorption of carboxyl groupwas recognized around 1,774.4 cm⁻¹ and a characteristic absorption of OHgroup was recognized around 3,557.0 cm⁻¹ and 3,087.7 cm⁻¹. A valueobtained from the equation (Sco/Scf)×(D/Dp)×(F/Fp) was 0.53.

The obtained fluorine-containing elastomer (carboxyl group-containingelastomer), 2,2-bis(3-amino-4-hydroxyphenyl) hexafluoropropane(bis(aminophenol) AF) as a crosslinking agent and carbon black (ThermaxN-990 available from Cancarb Co., Ltd.) as a filler were mixed in aweight ratio of 100/2/20, and the mixture was kneaded with an open rollto give a crosslinkable fluorine-containing rubber composition.

After the fluorine-containing rubber composition was subjected topressing at 180° C. for 10 minutes for crosslinking, further atwo-staged crosslinking in an oven was carried out under the conditionsmentioned in Table 4. Thus a 2 mm thick crosslinked product and a sampleO-ring (AS-568A-214) were produced. The results of measurements of thecrosslinked product with respect to crosslinkability, physicalproperties in ordinary state and compression set are shown in Table 4.

EXAMPLE 3

A crosslinkable fluorine-containing rubber composition was prepared inthe same manner as in Example 2 except that a bisdiaminophenyl compoundrepresented by the formula:

was used instead of bis(aminophenol) AF as a crosslinking agent. Acrosslinking was carried out under the conditions mentioned in Table 4(the same conditions as in Example 1). Crosslinkability, physicalproperties in ordinary state and compression set of the obtainedcrosslinked product were measured in the same manner as in Example 1.The results are shown in Table 4.

EXAMPLE 4

(Triazine Crosslinking System)

A crosslinked product was produced under the conditions shown in Table 4in the same manner as in Example 1 except that 3 parts by weight oftetraphenyltin was blended as a crosslinking agent instead ofbis(aminophenol) AF and 10 parts by weight of SRF carbon black wasblended based on 100 parts by weight of the nitrile group-containingelastomer having carboxyl group at an end thereof and obtained inExample 1. Crosslinkability and each physical property of thecrosslinked product were measured in the same manner as in Example 1.The results are shown in Table 4.

TABLE 4 Example 1 Example 2 Example 3 Example 4 Components CN-containingpolymer 100 — — 100 COOH-containing polymer — 100 100 — Tetraphenyltin —— — 3 Bis(aminophenyl) AF 2 2 — — Tetraamine — — 2 — MT carbon 20 20 20— SRF carbon — — — 10 Vulcanizability 180° C. 210° C. 180° C. 200° C. νmin (kgf) 0.8 0.2 0.5 0.4 ν max (kgf) 4.2 1.6 3.0 2.7 T10 (min) 2.4 1.71.6 0.3 T90 (min) 6.6 16.5 14.5 19.0 Vulcanization conditions Pressvulcanization 180° C. × 10 min 218° C. × 20 min 180° C. × 10 min 180° C.× 10 min Oven vulcanization 200° C. × 18 h + 204° C. × 18 h + 200° C. ×18 h + 204° C. × 18 h + 288° C. × 18 h 288° C. × 18 h 288° C. × 18 h288° C. × 18 h Physical properties in ordinary state 100% modulus MPa(kgf/cm²) 11.3 (115) 5.8 (59) 13.5 (138) 6.4 (65) Tensile strength MPa(kgf/cm²) 21.5 (219) 18.3 (187) 19.1 (195) 13.5 (138) Elongation (%) 150201 130 140 Hardness (JIS A) 75 72.9 82 70 Compression set (%) 200° C. ×70 h 6 37 12 19 200° C. × 168 h 9 — — 24 230° C. × 70 h 7 45 12 — 230°C. × 168 h 10 — — —

As shown in Table 4, a crosslinked product being excellent in physicalproperties in ordinary state and compression set can be obtained at alow crosslinking temperature from the nitrile group-containing polymerhaving carboxyl group at an end thereof (Example 1), and also acrosslinked product being excellent in physical properties in ordinarystate can be obtained from the carboxyl group-containing polymer byoxazole crosslinking (Example 2). Also in case of imidazole crosslinkingof the carboxyl group-containing polymer, particularly a crosslinkedproduct having improved compression set at a low crosslinkingtemperature (180° C.) is obtained (Example 3). Further even if thenitrile group-containing polymer having carboxyl group at an end thereofis crosslinked with tetraphenyltin (Example 4), a minimum viscosity atcrosslinking is low and good processability is exhibited.

INDUSTRIAL APPLICABILITY

According to the present invention, a novel fluorine-containingelastomer to which a new crosslinking system can be applied can beobtained, and by crosslinking the elastomer, a mechanical strength andcompression set, particularly compression set at high temperature can beenhanced greatly.

1. A process for preparing a fluorine-containing elastomer having acarboxyl group at an end of a trunk chain and/or branched chain of theelastomer as a crosslinking group, comprising the steps of: polymerizinga fluorine-containing monomer using a compound giving free carboxylgroup to the trunk chain and/or branched chain of the elastomer, whichcompound comprises at least one of a polymerization initiator and amonomer, to obtain a polymerization product, treating the polymerizationproduct with an acid, and isolating the elastomer from thepolymerization product.
 2. The process according to claim 1, wherein thefluorine-containing elastomer has free carboxyl group at an end of atrunk chain as a crosslinkable group.
 3. The process according to claim1, wherein the fluorine-containing monomer is polymerized by an emulsionpolymerization method.
 4. The process according to claim 1, wherein saidtreating step comprises converting a group comprising a metal salt andan ammonium salt of carboxylic acid, as may be present in thepolymerization product, to carboxyl group.
 5. The process according toclaim 1, wherein the isolating step is conducted by separating theelastomer which is coagulated through the acid treatment step.
 6. Theprocess according to claim 1, wherein the isolating step is conducted bymeans of freeze coagulation.